This invention relates to an acoustic echo suppression circuit and, more particularly, to an art of acoustic echo suppression for cancelling residual echo signals which have been left unremoved by an echo canceller.
A conventional hands-free communication unit using a microphone and a loud speaker has been applied to a wide variety of fields such as a speaker phone system, a teleconferencing system, a phone conference system, cellular phone system used in an automobile or the like.
The hands-free communication unit allows two parties to communicate with each other bilaterally by collecting a talker's voice signal through a microphone and amplifying the received voice signal through a loud speaker. The received voice signal amplified through the loud speaker is likely to be collected by the microphone, causing the howling effect. Additionally an acoustic echo is caused by reflection of the talker's or receiver's voice or transmission delay under a certain communication environment. Therefore this hands-free communication unit requires to eliminate such howling effect as well as removing the acoustic echo.
The Japanese Patent Publication Laid-Open No. 14476 (1993) titled "Speaker Phone Unit" discloses a hands-free communication unit using an echo canceller for removing the acoustic echo and howling effect. The above speaker phone unit comprises an acoustic echo/howling canceller for removing an acoustic echo and howling effect, respectively and a channel echo/howling canceller constructed in the same way as in the acoustic echo/howling canceller. The acoustic echo/howling canceller removes acoustic echoes of the voice signals collected by a microphone. The channel echo/howling canceller removes channel echo and howling caused by wraparound of a hybrid circuit. As a result, this system keeps its communication quality from deterioration caused by a channel echo resulted from the received voice signal returning to a hybrid circuit and a peripheral noise such as the noise accompanied with running of the automobile, external noise or background noise.
The above speaker phone unit assumes transfer characteristics of the acoustic echo based on the received voice signal through the acoustic echo canceller and cancels the acoustic echo of the sounds collected by the microphone. For example, in the cellular phone system in an automobile, when the peripheral noise such as a wind blast accompanied with running of the automobile becomes louder, the microphone collects such peripheral noise as well as the acoustic echo therein, which is input to the acoustic echo canceller. The acoustic canceller fails to cancel the signal having transfer characteristics other than those assumed, thus leaving the residual echo unremoved and deteriorating communication quality.